This invention relates to a fixing device of an induction heating type for use in an image forming apparatus such as a copying machine, a printer, and a facsimile machine and an image forming apparatus equipped with this fixing device.
Up to now, a fixing device for use in an image forming apparatus such as a copying machine, a printer, and a FAX machine has a structure such that a heating roller for heating a transfer material carrying a toner image composed of a powder developer and a pressing roller for conveying and pressing it are provided, and the toner particles on this transfer material are fused and melted to be fixed on it by the passage of the transfer material through the fixing point which is the pressing contact portion (nip portion) of these heating roller and pressing roller.
A generally known fixing device uses a halogen lamp, which is provided inside a metallic roller to heat the roller by radiant heat, as a heat source of the heating roller. However, in recent years, as a fixing device employing a technique of induction heating, there has been proposed a method of heating in which an eddy current is made to flow in a fixing roller in a structure having a shape composed of a coil supporting member provided along the rotary axis of the fixing roller having a heat applying coil wound round on it.
Such a fixing device is capable of making the position of heat generation come as close as possible to toner particles by generating an eddy current in the fixing roller by a magnetic flux to generate heat based on Joule""s heat, and is also capable of achieving improved efficiency of energy consumption as compared to a heating roller employing a halogen lamp. In addition, such a fixing device is capable of performing temperature control easily and with a high precision.
However, as shown in FIG. 28(a) and FIG. 28(b), in the case where a heat applying coil 101 is uniformly wound around a coil supporting member 100, as shown in FIG. 29, the magnetic flux density is small at both ends of the fixing roller in the longitudinal direction. As a result, heat generation is decreased to cause a temperature drop to occur, which produces poor fixing.
By making the length of the fixing roller longer than the part that is required for fixing in order to get rid of the problem of this temperature drop, the temperature of the part that is required for fixing is secured. In that case, however, the length of the heat applying coil becomes long, electric power consumption becomes high, and it is not possible to make the size compact, which causes the device to have a large size. Further, there is a problem that, for example, manufacturing cost is increased because the heat applying coil and the supporting member must be made large-sized.
This invention was made in view of the above-mentioned points, and it is an object to provide a fixing device and an image forming apparatus which are capable of securing the heat generation area with a simple structure, and which can be made small in size and at a low manufacturing cost.
For the purpose of solving the above-mentioned problems and accomplishing the object, this invention has any one of the structures described below.
A fixing device according to a first aspect of the invention comprises a cylindrical fixing roller to be heated, a coil supporting member disposed along the inside of this fixing roller, a heat applying coil formed by winding a wire round on the circumference of this coil supporting member for heating the fixing roller by inducing an induced current in the fixing roller, and a pressing roller that forms a nip portion with the fixing roller, conveys a transfer material which is gripped by the nip portion between the fixing roller and the pressing roller, and a toner image formed on the transfer material being fixed by heat energy given from the fixing roller, characterized in that the heat applying coil is formed by winding a wire round in such a way as to have a shape which is longer in the longitudinal direction of the coil supporting member and parts parallel to the longitudinal direction of the coil supporting member and parts lying along the circumference of the coil supporting member, and with respect to this heat applying coil, the winding density of the parts lying along the circumference of the coil supporting member is made higher than the winding density of the parts parallel to the longitudinal direction of the coil supporting member.
According to this aspect of the invention, by making the winding density of the parts of the heat applying coil lying along the circumference of the coil supporting member higher than the winding density of the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member, the magnetic flux density at the end portions in the longitudinal direction of the fixing roller is made higher, and heat generation becomes higher accordingly; thus, the distribution of heat generation in the fixing roller is improved thereby making possible to suppress the temperature drop at the end portions in the longitudinal direction, and it is possible that the length of the heat applying coil is made as short as possible, a sufficient heat generation area is secured with a simple structure, and the size is made small and the manufacturing cost is lowered.
According to a second aspect of the invention, the wire of the heat applying coil has a cross-sectional shape such that the lengths of the vertical axis and the lateral axis intersecting each other perpendicularly at the center are different, and the heat applying coil is formed by winding the wire round in such a way that, in the parts parallel to the longitudinal direction of the coil supporting member, the long axes of the wire cross-section are directed to one another, and in the parts lying along the circumference of the heat applying coil, the short axes of the wire cross-section are directed to one another.
According to the second aspect of the invention, by forming the coil in such a way that, in the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member, the long axes of the wire cross-section are directed to one another, and in the parts lying along the circumference of the coil supporting member, the short axes of the wire cross-section are directed to one another, the magnetic flux density at the end portions in the longitudinal direction of the fixing roller is made higher, and heat generation becomes higher accordingly; thus, the distribution of heat generation in the fixing roller is improved thereby making it possible to suppress the temperature drop at the end portions in the longitudinal direction, and it is possible that the length of the heat applying coil is made as short as possible, a sufficient heat generation area is secured with a simple structure, and the size is made small and the manufacturing cost is lowered.
According to a third aspect of the invention, a fixing device is provided wherein, with respect to the heat applying coil, the number of stacked layers are different between the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member and the parts of the heat applying coil lying along the circumference of the coil supporting member, and the number of stacked layers in the parts of the heat applying coil lying along the circumference of the coil supporting member is made greater than the number of stacked layers in the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member.
According to the third aspect of the invention, by making the number of stacked layers in the parts of the heat applying coil lying along the circumference of the coil supporting member greater than the number of stacked layers in the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member, the magnetic flux density at the end portions in the longitudinal direction of the fixing roller is made higher, and heat generation becomes higher accordingly; thus, the distribution of heat generation in the fixing roller is improved thereby making it possible to suppress the temperature drop at the end portions in the longitudinal direction, and it is possible that the length of the heat applying coil is made as short as possible, a sufficient heat generation area is secured with a simple structure, and the size is made small and the manufacturing cost is lowered.
According to a fourth aspect of the invention, a fixing device is provided wherein, with respect to the heat applying coil, the distance between the outer surface of the wire in the parts of the heat applying coil lying along the circumference of the coil supporting member and the inner surface of the fixing roller is made shorter than the distance between the outer surface of the wire in the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member and the inner surface of the fixing roller.
According to the fourth aspect of the invention, by making the distance between the outer surface of the wound wire in the parts of the heat applying coil lying along the circumference of the coil supporting member and the inner surface of the fixing roller shorter than the distance between the outer surface of the wound wire in the parts of the heat applying coil parallel to the longitudinal direction of the coil supporting member and the inner surface of the fixing roller, the magnetic flux density at the end portions in the longitudinal direction of the fixing roller is made higher, and heat generation becomes higher accordingly; thus, the distribution of heat generation in the fixing roller is improved thereby making it possible to suppress the temperature drop at the end portions in the longitudinal direction, and it is possible that the length of the heat applying coil is made as short as possible, a sufficient heat generation area is secured with a simple structure, and the size is made small and the manufacturing cost is lowered.
According to a fifth aspect of the invention, the lead wires of the heat applying coil are disposed inside the winding of the heat applying coil.
According to the fifth aspect of the invention, by disposing the lead wires of the heat applying coil inside the winding of the heat applying coil, the lead wires do not stand in the way of the heat applying coil to be wound round, and on top of it, they are easily connected to a power source.
According to a sixth aspect of the invention, the coil supporting member is heat resistant, and its both ends are supported fixedly.
According to the sixth aspect of the invention, by making the coil supporting member heat resistant and its both ends supported fixedly, the durability of the coil supporting member is improved.
According to a seventh aspect of the invention, the lead wires of the heat applying coil are fixed to the coil supporting member, and are drawn out from one side of the coil supporting member in the longitudinal direction, to be connected to a power source.
According to the seventh aspect of the invention, by making the lead wires of the heat applying coil fixed to the coil supporting member and drawn out from one side of the coil supporting member in the longitudinal direction, the lead wires can be connected to a power source simply and certainly, and on top of it, the efficiency of operation in assembly is improved.
According to an eighth aspect of the invention, a heat insulating layer is formed between the heat applying coil and the fixing roller.
According to the eighth aspect of the invention, by forming a heat insulating layer between the heat applying coil and the fixing roller, heat radiation from the fixing roller is decreased, which makes possible stable fixing and reduction of power consumption.
According to a ninth aspect of the invention, the coil supporting member is a heat conducting material and has means for radiating heat of the heat applying coil.
According to the ninth aspect of the invention, by making the coil supporting member thermally conductive to radiate heat of the heat applying coil, the durability of the coil supporting member and the heat applying coil can be improved.
According to a tenth aspect of the invention, the coil supporting member is composed of half-divisional parts which are obtained by dividing the coil supporting member into two parts along its longitudinal direction, two parts of the heat applying coil are formed by winding wires round on the respective half-divisional parts from inner side to the outer side in such a way that the terminals at the start of winding come to the inner side, the both half-divisional parts are bonded to each other, and the one and the other of the ends of winding at the outside of the heat applying coils are connected to each other.
According to the tenth aspect of the invention, by winding round wires of the heat applying coil on the half-divisional parts respectively from the inner side to the outer side, in such a way that the terminals at the start of winding come to the inner side, to build two winding units, bonding the both half-divisional parts to each other, and connecting the one and the other of the ends of winding at the outer side of the heat applying coils to each other, they are easily built and the efficiency of the operation of assembly is improved.
According to an eleventh aspect of the invention, an image forming apparatus is provided in which a uniformly charged photoreceptor drum is imagewise exposed, a latent image is formed on the photoreceptor drum, this latent image is developed to form a toner image, this toner image is transferred onto a transfer material, and the toner image is fixed, characterized in that the image forming is equipped with one of the above-described fixing devices to fix the toner image on the transfer material.
According to the eleventh aspect of the invention, by being equipped with a fixing device that is small-sized, of low cost, and capable of securing a sufficient heat generating area, the image forming apparatus can also be made small-sized and of low cost.